On 06/13/2018 03:15 AM, Michal Hocko wrote: > On Wed 13-06-18 08:32:19, Vlastimil Babka wrote: >> On 06/12/2018 04:11 PM, Jason Baron wrote: >>> >>> >>> On 06/12/2018 03:46 AM, Michal Hocko wrote: >>>> On Mon 11-06-18 12:23:58, Jason Baron wrote: >>>>> On 06/11/2018 11:03 AM, Michal Hocko wrote: >>>>>> So can we start discussing whether we want to allow MADV_DONTNEED on >>>>>> mlocked areas and what downsides it might have? Sure it would turn the >>>>>> strong mlock guarantee to have the whole vma resident but is this >>>>>> acceptable for something that is an explicit request from the owner of >>>>>> the memory? >>>>>> >>>>> >>>>> If its being explicity requested by the owner it makes sense to me. I >>>>> guess there could be a concern about this breaking some userspace that >>>>> relied on MADV_DONTNEED not freeing locked memory? >>>> >>>> Yes, this is always the fear when changing user visible behavior. I can >>>> imagine that a userspace allocator calling MADV_DONTNEED on free could >>>> break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We >>>> have the new flag much shorter so the probability is smaller but the >>>> problem is very same. So I _think_ we should treat both the same because >>>> semantically they are indistinguishable from the MADV_DONTNEED POV. Both >>>> remove faulted and mlocked pages. Mlock, once applied, should guarantee >>>> no later major fault and MADV_DONTNEED breaks that obviously. >> >> I think more concerning than guaranteeing no later major fault is >> possible data loss, e.g. replacing data with zero-filled pages. > > But MADV_DONTNEED is an explicit call for data loss. Or do I miss your > point? > >> The madvise manpage is also quite specific about not allowing >> MADV_DONTNEED and MADV_FREE for locked pages. > > Yeah, but that seems to describe the state of the art rather than > explain why. > >> So I don't think we should risk changing that for all mlocked pages. >> Maybe we can risk MCL_ONFAULT, since it's relatively new and has few users? > > That is what Jason wanted but I argued that the two are the same from > MADV_DONTNEED point of view. I do not see how treating them differently > would be less confusing or error prone. It's new so we can make it > behave differently is certainly not an argument. > >>>> So the more I think about it the more I am worried about this but I am >>>> more and more convinced that making ONFAULT special is just a wrong way >>>> around this. >>>> >>> >>> Ok, I share the concern that there is a chance that userspace is relying >>> on MADV_DONTNEED not free'ing locked memory. In that case, what if we >>> introduce a MADV_DONTNEED_FORCE, which does everything that >>> MADV_DONTNEED currently does but in addition will also free mlock areas. >>> That way there is no concern about breaking something. >> >> A new niche case flag? Sad :( >> >> BTW I didn't get why we should allow this for MADV_DONTNEED but not >> MADV_FREE. Can you expand on that? > > Well, I wanted to bring this up as well. I guess this would require some > more hacks to handle the reclaim path correctly because we do rely on > VM_LOCK at many places for the lazy mlock pages culling. > The point of not allowing MADV_FREE on mlock'd pages for me was that with mlock and even MLOCK_ON_FAULT, one can always can always determine if a page is present or not (and thus avoid the major fault). Allowing MADV_FREE on lock'd pages breaks that assumption. Thanks, -Jason
